Objects And Strings In Java
April 30, 2017
Categorised in: Java Core
About Objects
- Object in Java is an instance of a Java class.
- When you create a variable and point it at an object. The variable isn’t the object itself, instead it is just a reference to the object
- Non-primitive variables are references to objects.
- Objects can have multiple references.
- That is, they point to single object.
- For instance, in memory some values assigned to that object changes. This value change is reflected in all of its references.
Example
public class ObjectInJavaExample {
public String color;
public static void main( String[] args ){
ObjectInJavaExample candy = new ObjectInJavaExample();
candy.color = "Red";
candy.displayCandy();
}
private void displayCandy() {
System.out.println("Color of candy is: " +this.color);
}
}
Output: Color of candy is: Red
Notes:
- The String variable does not have static. This means it is an instance varaible or field.
- An instance variable is not a member of the class itself. It is member of an instance of class.
- In main method, candy is an instance of ObjectInJavaExample class. The data type is name of class.
- ObjectInJavaExample() is a call to constructor method.
- ObjectInJavaExample candy = new ObjectInJavaExample(); line is called as instantiation
- Now once the instance is declared, the variables of instance are allocated memory.
- Class instances or objects can also have instance methods. displayCandy() is an instance method.
- this keyword helps to output information of the instance of class which called the method.
String Is an Object In Java
- The String class, just like the helper classes of primitive data types, is member of java.lang.String.
- Example:
- String str = new String(“Hello World!”); //This creates a string variable
- String class is special because you can use a shortcut too:
- String str1 = “Hello World!”; //This is exactly same as before
- String is basically an array of characters. However, character is a primitive and string is a complex object.
- String can accommodate as many characters as you want till the memory is available.
- String objects are immutable.
- That is, once you instantiate and assign a value, you can’t change the value.
- To us, it looks like we are changing the value of same object.
- However, what happens in back-end is de-referencing.
- Example:
- String str1 = “Okay”;
- str1 = “Not Okay”; //De-referencing
- The old object with “Okay” is assigned for garbage collection.
- Thus, we create a brand new object with the new value.
Character Array to String
public class CharArrayToString {
public static void main( String[] args ){
char[] charArr = {'O', 'k','a','y'};
String str = new String(charArr); //using String constructor
System.out.println(str);
}
}
String to Character Array
public class StringToCharArray {
public static void main( String[] args ){
String str = "Hello World!";
char[] charResult = str.toCharArray();
for(char c : charResult){
System.out.print(c);
}
}
}
